We are witnessing continued progress in the miniaturization of mechanical structures such as arm assemblies for disk drives, bimorph beams and other constructions employing mechanical arms. This move to ever smaller physical dimensions has to be accompanied by corresponding advances in devices controlling the deflections or displacements of the arm and units which sense these movements, e.g., for feedback purposes. Thus, in the field of disk drives, fine-tuning of the displacement of the tip of the arm bearing the read/write head is very important. High-bandwidth servo actuators are crucial to achieving large track densities in rotating magnetic storage devices. General ways of achieving higher bandwidth include the use of high specific stiffness materials such as AlBC or Albumet for making the arms, employment of compound actuators and application of active/passive damping techniques.
The use of dual-stage actuation systems having a primary actuator, e.g., a VCM coil, for executing large movements and a secondary actuator, e.g., a piezoelectric element PZT, for fine-tuning and tracking is well-known in the art. The small PZT miliactuator has higher vibrational modes than the VCM due to scaling. Descriptions of such systems are found in scientific and patent literature. Some representative references include Ahi-Min Y., et al. "Controller Design Criteria for the Dual-Stage Disk Actuator System", Proc. SPIE--International Society for Optical Engineering (USA), Vol. 2101, No. 1, 1993, pp. 305-8 and Guo, W., et al., "Dual Stage Actuators for High Density Rotating Memory Devices", IEEE Trans. Magn. (USA), Vol .34, No. 2, pt. Mar. 1, 1998, pp. 450-5.
A PZT transducer is suitable for actuation purposes and is generally controlled by applying to it a suitable control voltage. In response, the PZT changes its physical dimensions causing a deflection or displacement of the arm. Additionally, certain PZTs such as piezoelectric ceramics are inexpensive and combine simultaneously actuation and sensing capabilities. In particular, such "smart" PZTs or self-sensing PZTs react to a change in mechanical load by changing their electrical input parameters. They can thus sense the deformation of the PZT or deflection of the arm while at the same time performing actuator functions. Self-sensing PZTs are described in prior art references such as Janocha H., et al. "Principle of Smart Piezoactuators", Actuator 96, 5th International Conference on New Actuators", Jun. 26-28, 1996, Bremen, Germany, and Near C. D., "Piezoelectric Actuator Technology", SPIE International Society for Optical Engineering, Vol. 2717, April 1998, pp. 246-58. Additional prior art references teach the use of PZTs as devices for active damping of vibrating structures or arms. These applications of PZTs are described, e.g., by W. W. Chiang, et al. in "Piezoelectric Modal Sensor/Actuator Devices for DASD Active Damping Vibration Control", IBM Technical Disclosure Bulletin, Vol. 34, No. 4B, September 1991, pp. 53-4 and by J. R. Kirtley, et al. in "Active Vibration Damping of Scanning Tunneling Microscope", IBM Technical Disclosure Bulletin, Vol. 31, No. 2, July 1988, pp. 426-9.
A PZT element used as a miliactuator for deflection control as well as sensing has to be compensated for external influences of temperature, moisture and any electrical fields. Lack of proper compensation of the miliactuator will result in inferior sensing performance or even make it impossible to separate the driving signal causing the deformation from the self-sensing signal. Only when the self-sensed signal is accurate can it be used for compensating the arm assembly, e.g., compensation of vibrational modes.